All solid state Thin Film Batteries (TFB) are known to exhibit several advantages over conventional battery technology such as superior form factors, cycle life, power capability and safety. However, there is a need for cost effective and high-volume manufacturing (HVM) compatible fabrication technologies to enable broad market applicability of TFBs.
Most of the past and current state-of-the-art approaches, as they pertain to TFB and TFB fabrication technologies, have been conservative, wherein the efforts have been limited to scaling the basic technologies of the original Oak Ridge National Laboratory (ORNL) device development that started in the early 1990s. A summary of ORNL TFB development can be found in N. J. Dudney, Materials Science and Engineering B 116, (2005) 245-249.
FIGS. 1A to 1F illustrate a traditional process flow for fabricating a TFB on a substrate. In the figures, a top view is shown on the left and a corresponding cross-section A-A is shown on the right. There are also other variations, e.g., an “inverted” structure, wherein the anode side is grown first, which are not illustrated here. FIG. 2 shows a cross-sectional representation of a complete TFB, which may have been processed according to the process flow of FIGS. 1A to 1F.
As shown in FIGS. 1A and 1B, processing begins by forming the cathode current collector (CCC) 102 and anode current collector (ACC) 104 on a substrate 100. This can be done by (pulsed) DC sputtering of metal targets (˜300 nm) to form the layers (e.g. main group metals such as Cu, Ag, Pd, Pt and Au, metal alloys, metalloids or carbon black), followed by masking and patterning for each of the CCC and ACC structures. It should be noted that if a metallic substrate is used, then the first layer may be a “patterned dielectric” deposited after a blanket CCC 102 (the CCC may be needed to block Li in the cathode from reacting with the substrate). Furthermore, the CCC and ACC layers may be deposited separately. For example, the CCC may be deposited before the cathode and the ACC may be deposited after the electrolyte, as shown in FIG. 3. For current collector layers formed of metals such as Au and Pt that do not adhere well to, for example, oxide surfaces, adhesion layers of metals such as Ti and Cu can be used.
Next, in FIGS. 1C and 1D, the cathode 106 and electrolyte layers 108 are formed, respectively. RF sputtering has been the traditional method for depositing the cathode layer 106 (e.g. LiCoO2) and electrolyte layer 108 (e.g. Li3PO4 in N2). However, pulsed DC has been used for LiCoO2 deposition. The cathode layer 106 can be a few to several or more microns thick, and the electrolyte layer 108 can be about 1 to 3 μm thick.
Finally, in FIGS. 1E and 1F, the Li layer 110 and protective coating (PC) layer 112 are formed, respectively. The Li layer 110 can be formed using an evaporation or a sputtering process. The Li layer 110 can be a few to several or more microns thick (or other thickness depending on the thickness of the cathode layer) and the PC layer 112 can be in the range of 3 to 5 μm, and more depending on the materials constituting the layer. The PC layer 112 can be a multilayer of parylene (or other polymer-based material), metal or dielectric. Note that, between formation of the Li layer and the PC layer, the part must be kept in an inert or reasonably inert environment, such as argon gas or dry-room conditions.
There may be an additional “barrier” layer deposition step, prior to the CCC 102, if the CCC does not function as the barrier and if the substrate and patterning/architecture call for such a barrier layer. Also, the protective coating need not be a vacuum deposition step.
In typical processes, annealing of the cathode layer 106 will be required in order to improve the crystallinity of the layer if the TFB performance specification calls for “plateau of operating voltage”, high power capability and extended cycle life.
While some improvements have been made to the original ORNL approaches, there are many problems with the prior art fabrication processes for TFBs that prevent them from being compatible with cost effective and high-volume manufacturing (HVM), and thereby preclude broad market applicability of TFBs. For example, issues with the state-of-the-art thin film cathode and cathode deposition processes include: (1) a low deposition rate leading to low throughput and inefficient scaling (of economy) for cost reduction, and (2) a need for a high temperature anneal for the crystalline phase, which adds to process complexity, low throughput and limitations on the choice of substrate materials.
Accordingly, a need remains in the art for fabrication processes and technologies for TFBs that are compatible with cost effective and high-volume manufacturing (HVM), and thereby enable broad market applicability of TFBs.